function x=TowDerivation(Original,location,error)
    spacing = 0.001;
    tmp = error + 1;
    a = 0;   b = 0;
    while(tmp > error)
        a = Original(location) - 2* Original(location - spacing) + Original(location - 2* spacing) ...
            / 2*spacing; 
        b = Original(location +2* spacing) - 2* Original(location + spacing) + Original(location) ...
            / 2*spacing; 
        tmp = abs(a-b); 
        spacing = spacing/2;
    end
    x = (a + b) / 2;
end